2 Answers
2

As a Super Admin, you have full permission (view/edit) to each calendar in the domain. Therefore, to get access to the user's primary calendar, just enter the user's email address in the "Add a coworker's calendar" box (underneath "Other Calendars").

Note that this trick will only work for a user's primary calendar. If a user has created an additional calendar, you'll have to find that calendar (I suggest using the GAM (free third-party tool) command gam user bob@acme.com show calendars - the ID value can be pasted into the "Add a coworker's calendar" box so you can view it or added to your account with the command gam calendar <calendar ID> add editor <your email>.

User the account that still has access to the calendar.
Go to Calendar settings and click "HTML" to get the URL of the calendar.
Copy that URL
Sign into your admin account
Paste and go to the calendar URL
click "+ Google Calendar" button in bottom right of the calendar.

I really wish Google would give you a way to see all active calendars within a domain. Maybe one day....